This invention relates to turbo machinery apparatus and methods. More particularly, this invention relates to a combustion turbine engine having a turbine section including a stator assembly directing a flow of motive fluid onto a rotatable turbine wheel. The turbine stator assembly uniquely provides a variable flow area for the flow of motive fluid to thereby efficiently accommodate varying engine operating conditions. Thus, this invention relates particularly to a method of operating a combustion turbine engine.
A long-recognized need in the field of turbo machinery has been to provide variable-area turbine stator assemblies for those turbo machines intended to operate under transient or non-steady state conditions. For example, combustion turbine engines designed as aerospace propulsion units may be required to fulfill a variety of mission objectives. Among these missions objectives may be engine operation under take-off, climb, subsonic cruise, supersonic cruise, loiter, and combat power settings at a variety of altitudes and with relatively high engine fuel efficiencies specified for each of the various operational modes. Because the mass air flow rate through the engine varies with the engine operating condition, the turbine stator must define a variable area in order to direct the varying flow of motive fluid onto the turbine wheel to produce best turbine wheel operating efficiency. Engine efficiency is directly influenced by the efficiency of the turbine wheel operation.
A conventional combustion turbine engine having variable stator vanes is known in accordance with U.S. Pat. No. 3,237,918, granted Mar. 1, 1966 to C. E. LeBell, et al., wherein the turbine stator includes stator vanes having a leading edge portion extending radially between a pair of concentric annular walls. The annular walls are generally cylindrical in the vicinity of the stator vanes and bound an annular flow path for the motive fluid. The stator vanes each include a movable trailing edge portion and a flexible midsection connecting the leading and trailing edge portions. An actuator mechanism extending into each stator vane connects with the movable trailing edge portions. By moving the trailing edge portions relative to the leading edge portions, the flow area through the turbine stator is variable.
A combustion turbine engine having a turbine stator assembly according to the Le Bell, et al., invention has a number of recognized deficiencies. Among these deficiencies is the fact that each stator vane is a complex assembly of component parts. Consequently, the turbine stator assembly is complex and expensive to manufacture. Further, the flexible mid-section of each stator vane must flex within the hot motive gas flow each time the area of the turbine stator assembly is changed. As a result, the stator vanes may be prone to failure and may require frequent maintenance. Moreover, because the walls which bound the flow path are cylindrical in the vicinity of the stator vanes, the walls do not define an optimally-shaped flow path for the motive fluid upstream of the turbine wheel. Thus, the efficiency of the turbine wheel and of the engine may be decreased because the motive fluid is not directed onto the turbine wheel to produce optimum effect. Still further, because there exists a considerable fluid pressure difference between the suction or convex side of the stator vanes and the pressure or concave side of the vanes, motive fluid leaks between the ends of the stator vanes and the walls which bound the flow path. Experience has shown that this leakage alone is responsible for a major part of the efficiency loss of conventional variable-area stator assemblies. Consequently, it has been proposed to provide sealing members at the ends of the stator vanes which would sealingly and movably engage the walls of the flow path. However, the use of such sealing members requires that the walls of the flow path be generally cylindrical in the vicintiy of the sealing members, so that a non-optimum flow path shape results and limits engine efficiency.
In light of the many recognized deficiencies of conventional variable-area turbine stators the results of recent studies are all the more perplexing. These studies of future aerospace applications for combustion turbine engines indicate a need for an engine having a variable-area turbine stator capable of about a 30 percent area variation. This 30 percent area variation is significantly greater than that achieved by conventional variable-area stators. It is expected that an attempt to build a conventional variable-area turbine stator capable of a 30 percent area variation would result in an exacerbation of the recognized deficiencies of the conventional stator and an unsatisfactory engine performance.